1. Field of the Invention
The present invention relates to a non-oriented electromagnetic steel sheet which is advantageous for electric materials used for electric appliances, and to a method for producing the same.
2. Description of the Related Arts
Electromagnetic steel sheets with less iron loss have been desired in recent years from energy saving point of view of electric appliances. Since coarsening of crystal grains is effective for decreasing iron loss, it is attempted in the middle and high grade non-oriented electromagnetic steel sheets, which are especially required to have low iron loss values, containing 1 to 3% of (Si+Al) to coarsen crystal grains by increasing the finish anneal temperature up to 1000.degree. C. or by lowering the line speed for annealing to prolong the annealing time.
It is effective for desirable grain growth during the finish annealing to diminish the content of impurities and precipitates in the steel sheet. For this purpose, many attempts have been made to lend impurities and precipitates harmless, especially to decrease S content in order to prevent MnS from precipitating in high glade materials.
Japanese Examined Patent Publication No. 56-22931 discloses, for example, an art for decreasing S content and O content to 50 ppm or less and 25 ppm or less, respectively, in order to decrease iron loss in the steel containing 2.5 to 3.5% of Si and 0.3 to 1.0% of Al.
Japanese Examined Patent Publication No. 2-50190 also discloses an art for decreasing S content, O content and N content to 15 ppm or less, 20 ppm or less and 25 ppm or less, respectively, in order to decrease iron loss in the steel containing 2.5 to 3.5% of Si and 0.25 to 1.0% of Al.
Japanese Unexamined Patent Publication No. 5-140647 further discloses an art for decreasing S content to 30 ppm or less, and Ti, Zr, Nb and V contents to 50 ppm or less, respectively, in order to decrease iron loss in the steel containing 2.0 to 4.0% of Si and 0.10 to 2.0% of Al.
However, it is the current situation that the iron loss value of the high grade steel sheet with S content of 10 ppm or less is in the order of W.sub.15/50 =2.4 W/kg (with a sheet thickness of 0.5 mm) and the iron loss values lower than this value have not been attained. The iron loss seems to be simply decreased more and more because MnS content is diminished accompanied by the decrease of the S content to facilitate crystal grain growth. However, the iron loss value described above is actually in its limit because decrease of the iron loss due to reduced S content will be saturated at a S content of about 10 ppm.